1. Field of the Invention
The present invention is designed to appropriately perform temperature compensation of a plastic lens applied to a scanning optical system or a multi-beam scanning optical system and, more particularly, to reduce defocus and a shift in scanning position even in a scanning optical system or a multi-beam scanning optical system using an oblique incident scheme in a laser beam printer apparatus.
2. Related Background Art
FIG. 1 is a sectional view showing a conventional scanning optical system using a single laser beam projected along the subscanning direction. FIG. 2 is a sectional view showing a multi-beam scanning optical system using a plurality of laser beams projected along the subscanning direction.
In FIG. 1, a parallel beam (collimated beam) emerging from a laser collimator unit (not shown) is focused to a deflection point P near the deflection plane of an optical deflector 65 comprising, e.g., a polygon mirror through an incident lens 60 serving as an incident optical system. The laser beam deflectively reflected by the deflection plane is focused onto a target irradiation surface 66 through an f-.theta. lens 63 serving as an imaging optical system having a focusing function and f-.theta. characteristics so that the target irradiation surface 66 is scanned with the laser beam along the main scanning direction. In FIG. 1, the deflection point P and a focal point Q on the target irradiation surface 66 are optically conjugate with each other with respect to the f-.theta. lens 63 within the subscanning section.
The scanning optical system having this arrangement is used for, e.g., a laser beam printer apparatus (LBP) or a digital copying machine. In this case, the irradiation object is a photosensitive body. A latent image formed on the photosensitive body is printed on a paper sheet or the like by a generally known electrophotographic process.
In FIG. 2, two parallel beams (collimated beams) A and B emerging from two laser collimator units (not shown) are focused to a deflection point P near the deflection plane of an optical deflector 75 comprising a polygon mirror through incident lenses 70A and 70B serving as corresponding incident optical systems, respectively, and are focused to two focal points (exposure positions) Q.sub.A and Q.sub.B on a target irradiation surface 76 through f-.theta. lenses 71A and 71B serving as corresponding imaging optical systems, respectively. With this operation, the target irradiation surface 76 is simultaneously scanned with the two laser beams A and B.
In FIG. 2, since the two laser beams A and B are focused to very close positions on the deflection plane, the polygon mirror can be made thin. As a result, the load on the motor for rotating the polygon mirror can be reduced, so the operation speed can be increased.
Each of the incident optical systems 70A and 70B shown in FIG. 2 makes the laser beam obliquely incident on the deflection plane of the optical deflector 75 within the subscanning section parallel to the drawing surface and is called an oblique incident optical system.
To correct aberration generated when the laser beam is obliquely incident on the polygon mirror, the f-.theta. lenses 71A and 71B must be decentered from the principal rays of the laser beams A and B from the deflection plane within the subscanning section. In addition, the interval between the focal points Q.sub.A and Q.sub.B on the target irradiation surface 76 is defined by the specifications of the product. For a resolution of, e.g., 600 dpi, the interval is set to be an odd multiple of 42.3 .mu.m, as is known.
When the interval between the focal points Q.sub.A and Q.sub.B is increased, and an additional electrophotographic process unit is disposed between the focal points Q.sub.A and Q.sub.B, a two-color copy image can be obtained.
The f-.theta. lens 63, 71A, or 71B as an imaging optical system shown in FIG. 1 or 2 is increasingly formed from a plastic lens in recent years because of the following advantages.
(1) Since the imaging optical system can be formed by one lens, unlike a glass lens, the entire apparatus can be made compact and lightweight.
(2) Since the lens can be manufactured by molding, a large cost reduction and mass production are possible.
However, in the plastic lens, the change in the refractive index of the material or the shape due to an environmental variation (particularly a change in temperature) is larger than that of a glass lens by one or more orders of magnitudes, as is known. Consequently, the focal position of the laser beam on the target irradiation surface is shifted. Conventionally, the focal depth of the laser spot is set to be sufficiently large to cope with this phenomenon. In recent years, however, along with an increase in resolution of business equipment, a demand has arisen for an increase in stop diameter for making the laser spot small, so it is difficult to ensure a sufficiently large focal depth.
As a technique of coping with this requirement, the incident optical system 60 having a refracting power only within the subscanning section is constituted by a plano-convex cylindrical lens (glass plano-convex lens) 61 consisting of a glass material and a plano-concave cylindrical lens (plastic plano-concave lens) 62 consisting of a plastic material, as shown in FIG. 1, and caused to function as a temperature compensation system for the f-.theta. lens 63 consisting of a plastic material.
Assume that, upon an ambient temperature rise, (1) the refractive index of the material of the f-.theta. lens 63 decreases, and (2) the lens expands to relax a curvature R. The power of the f-.theta. lens 63 decreases in both the cases (1) and (2), so the focal point Q on the target irradiation surface 66 is defocused to Q'.
On the other hand, the absolute value of the power of the plastic plano-concave lens 62 of the incident lens 60 becomes small (the variation in the refractive index of the glass plano-convex lens 61 is much smaller than that of the plastic plano-concave lens 62). For this reason, the positive power of the entire incident lens 60 increases, so the deflection point P of the collimated beam is defocused to P'.
Therefore, when the respective elements are optimized such that the focal point P' after defocus and the focal point Q on the target irradiation surface 66 have an imaging conjugate relationship through the f-.theta. lens 63 upon a temperature rise, temperature compensation associated with defocus can be achieved.
The reason why the incident lens 60 is constituted by the glass plano-convex lens 61 and the plastic plano-concave lens 62 from the incident side in FIG. 1 is that spherical aberration can be easily corrected, and the glass plano-convex lens 61 can be easily manufactured.
However, the multi-beam scanning optical system using the oblique incident scheme as shown in FIG. 2 has the following problem. The f-.theta. lenses (71A and 71B) in FIG. 2 are optical systems symmetrical about a symmetrical axis M, and the following description will be provided of using the laser beam (light beam) passing through the f-.theta. lens 71B, as will be described below.
Upon a temperature rise as an environmental variation, the power of the f-.theta. lens 71B consisting of a plastic material decreases, so that the focal point Q.sub.B on the target irradiation surface 76 at room temperature moves to Q'.sub.B, i.e., defocus takes place. To correct this defocus, the oblique incident optical system 70B is constituted by a glass plano-convex lens and a plastic plano-concave lens as shown in FIG. 1 to change the deflection point P on the deflection plane at room temperature to a focal point P' after the temperature rise. With this arrangement, the focal point Q'.sub.B is also defocused and moves to Q".sub.B, so focus correction is enabled. More specifically, since the deflection point P moves to P' along the optical axis, the focal point Q'.sub.B also moves to Q".sub.B along the optical axis.
The focal point Q.sub.B at room temperature moves to Q".sub.B after temperature rise, so focus compensation is achieved. However, the spot position is shifted by the distance between the focal point Q.sub.B and the focal point Q". This positional shift of the spot is a serious problem in image formation because, especially in the multi-beam scanning optical system shown in FIG. 2, the interval between the focal points Q.sub.A and Q.sub.B of the two laser beams A and B on the target irradiation surface 76 changes before and after a temperature rise.